The Food and Drug Administration has fired off a warning letter to the vaping company Juul. The company is being warned that it is violating the law by marketing its products as a safer alternative to cigarettes. NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: Juul is allowed to sell its products to adults, but the FDA says it's not allowed to claim that they are safer than cigarettes. The company says in its marketing that its products improve the lives of people by being an alternative to cigarettes. And earlier this year, a congressional committee heard testimony from teenagers that a company representative at a school assembly said the product was, quote, "totally safe." Congressman Raja Krishnamoorthi, an Illinois Democrat, questioned teenager Philip Fuhrman, who recounted his experience with the Juul rep.

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RAJA KRISHNAMOORTHI: Did he say anything else about whether the government had approved Juul?

PHILIP FUHRMAN: He mentioned that the FDA was about to come out and say that Juul was 99% safer than cigarettes. And he said that that would happen very soon and that it was in FDA approval while the talk was going on.

HARRIS: The FDA warning letter cited that exchange as one example of illegal marketing of these nicotine delivery products. The letter goes on to explain that if a company wants to claim that vaping is a safer alternative to cigarettes, it needs to get the product approved under a specific section of the law. Juul has not done that. And in order to do so, the company would need to provide scientific evidence to back its claim.

Health experts note that these vaping products contain far less of the toxic chemicals such as tar that are found in cigarettes, but they do contain addictive nicotine, and the long-term consequences of vaping are unknown. Juul is the nation's biggest producer of vaping products and has previously come under fire for enticing a whole new generation to become hooked on nicotine. The company has 15 days to respond to the warning letter and says it will fully cooperate with the FDA.